spring源码剖析（四）自定义标签解析流程

spring源码剖析（四）自定义标签解析流程

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目录(?)[+]

 

解析总流程

 

 

自定义标签解析入口

 让我们先看BeanDefinitionParserDelegate类的parseCustomElement方法:

public BeanDefinition parseCustomElement(Element ele) {
    return parseCustomElement(ele, null);
}
//containingBd为父类bean,对顶层元素的解析应设置为null
public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
    String namespaceUri = getNamespaceURI(ele);
    //根据命名空间找到对应的NamespaceHandler进行解析
    NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
    if (handler == null) {
        error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
        return null;
    }
    //调用自定义的NamespaceHandler进行解析
    return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}

提取自定义标签处理器

跟踪this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);

进入DefaultNamespaceHandlerResolver中的resolve方法

/**
 * Locate the {@link NamespaceHandler} for the supplied namespace URI
 * from the configured mappings.
 * @param namespaceUri the relevant namespace URI
 * @return the located {@link NamespaceHandler}, or {@code null} if none found
 */
public NamespaceHandler resolve(String namespaceUri) {
    //获取所有已经配置的handler映射
    Map<String, Object> handlerMappings = getHandlerMappings();
    //根据命名空间找打对应信息
    Object handlerOrClassName = handlerMappings.get(namespaceUri);
    if (handlerOrClassName == null) {
        return null;
    }
    else if (handlerOrClassName instanceof NamespaceHandler) {
        //已做过解析的情况,直接从缓存读取
        return (NamespaceHandler) handlerOrClassName;
    }
    else {
        //没有做过解析,则返回的是类路径
        String className = (String) handlerOrClassName;
        try {
            //使用反射将类路径转换成类
            Class<?> handlerClass = ClassUtils.forName(className, this.classLoader);
            if (!NamespaceHandler.class.isAssignableFrom(handlerClass)) {
                throw new FatalBeanException("Class [" + className + "] for namespace [" + namespaceUri +
                        "] does not implement the [" + NamespaceHandler.class.getName() + "] interface");
            }
            //初始化类
            NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);
            //调用自定义的NamespaceHandler的初始化方法
            namespaceHandler.init();
            //记录在缓存
            handlerMappings.put(namespaceUri, namespaceHandler);
            return namespaceHandler;
        }
        catch (ClassNotFoundException ex) {
            throw new FatalBeanException("NamespaceHandler class [" + className + "] for namespace [" +
                    namespaceUri + "] not found", ex);
        }
        catch (LinkageError err) {
            throw new FatalBeanException("Invalid NamespaceHandler class [" + className + "] for namespace [" +
                    namespaceUri + "]: problem with handler class file or dependent class", err);
        }
    }
}

回忆上一篇的自定义标签使用中的MyNamespaceHandler便可以发现,其实上面的方法就是初始化了MyNamespaceHandler,然后调用了他的init()方法

/**
 *
 * @author Administrator
 */
public class MyNamespaceHandler extends NamespaceHandlerSupport {

    /* (non-Javadoc)
     * @see org.springframework.beans.factory.xml.NamespaceHandler#init()
     */
    @Override
    public void init() {
        registerBeanDefinitionParser("user", new UserBeanDefinitionParser());
    }

}

让我们在看看spring是如何获取handler映射的,查看getHandlerMappings()方法

 

/**
     * Load the specified NamespaceHandler mappings lazily.
     */
    private Map<String, Object> getHandlerMappings() {
        //如果没有被缓存,则开始进入缓存
        if (this.handlerMappings == null) {
            synchronized (this) {
                if (this.handlerMappings == null) {
                    try {
                        //this.handlerMappings在构造函数已被初始化为META-INF/Spring.handlers
                        Properties mappings =
                                PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);
                        if (logger.isDebugEnabled()) {
                            logger.debug("Loaded NamespaceHandler mappings: " + mappings);
                        }
                        Map<String, Object> handlerMappings = new ConcurrentHashMap<String, Object>(mappings.size());
                        //将properties文件合并到Map格式的handlerMappings中
                        CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings);
                        this.handlerMappings = handlerMappings;
                    }
                    catch (IOException ex) {
                        throw new IllegalStateException(
                                "Unable to load NamespaceHandler mappings from location [" + this.handlerMappingsLocation + "]", ex);
                    }
                }
            }
        }
        return this.handlerMappings;
    }

标签解析

得到了解析器和分析的元素侯,Spring就可以将解析工作委托给自定义解析器去解析了

让我们回到BeanDefinitionParserDelegate的handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));中

进入NamespaceHandlerSupport中的parse方法

/**
 * Parses the supplied {@link Element} by delegating to the {@link BeanDefinitionParser} that is
 * registered for that {@link Element}.
 */
public BeanDefinition parse(Element element, ParserContext parserContext) {
    //寻找解析器,并进行解析操作
    return findParserForElement(element, parserContext).parse(element, parserContext);
}

/**
 * Locates the {@link BeanDefinitionParser} from the register implementations using
 * the local name of the supplied {@link Element}.
 */
private BeanDefinitionParser findParserForElement(Element element, ParserContext parserContext) {
    //获取元素名称,也就是<myname:user 中的user,此时localname为user
    String localName = parserContext.getDelegate().getLocalName(element);
    //根据user找到对应的解析器也就是在
    //registerBeanDefinitionParser("user", new UserBeanDefinitionParser());注册的解析器
    BeanDefinitionParser parser = this.parsers.get(localName);
    if (parser == null) {
        parserContext.getReaderContext().fatal(
                "Cannot locate BeanDefinitionParser for element [" + localName + "]", element);
    }
    return parser;
}

让我们再跟踪到AbstractBeanDefinitionParser 的parse()方法

public final BeanDefinition parse(Element element, ParserContext parserContext) {
        AbstractBeanDefinition definition = parseInternal(element, parserContext);
        if (definition != null && !parserContext.isNested()) {
            try {
                String id = resolveId(element, definition, parserContext);
                if (!StringUtils.hasText(id)) {
                    parserContext.getReaderContext().error(
                            "Id is required for element '" + parserContext.getDelegate().getLocalName(element)
                                    + "' when used as a top-level tag", element);
                }
                String[] aliases = new String[0];
                String name = element.getAttribute(NAME_ATTRIBUTE);
                if (StringUtils.hasLength(name)) {
                    aliases = StringUtils.trimArrayElements(StringUtils.commaDelimitedListToStringArray(name));
                }
                //将AbstractBeanDefinition转换成BeanDefinitionHolder
                BeanDefinitionHolder holder = new BeanDefinitionHolder(definition, id, aliases);
                registerBeanDefinition(holder, parserContext.getRegistry());
                if (shouldFireEvents()) {
                    //需要通知监听器进行处理
                    BeanComponentDefinition componentDefinition = new BeanComponentDefinition(holder);
                    postProcessComponentDefinition(componentDefinition);
                    parserContext.registerComponent(componentDefinition);
                }
            }
            catch (BeanDefinitionStoreException ex) {
                parserContext.getReaderContext().error(ex.getMessage(), element);
                return null;
            }
        }
        return definition;
    }

虽然说是对自定义配置文件的解析,但是我们可以看到,在这个函数中大部分的大马士用来处理将解析后的AbstractBeanDefinition转换为BeanDefinitionHolder并注册的功能,而真正去做解析的事情委托了给parseInternal,真是这句代码调用了我们的自定义解析函数。在parseInternal中，并不是直接调用自定义的doParse函数，而是惊醒了一些列的数据准备，包括对beanClass，scope，lazyInit等属性的准备。

接下来，让我们看看parseInternal方法，先跟踪到AbstractSingleBeanDefinitionParser

/**
 * Creates a {@link BeanDefinitionBuilder} instance for the
 * {@link #getBeanClass bean Class} and passes it to the
 * {@link #doParse} strategy method.
 * @param element the element that is to be parsed into a single BeanDefinition
 * @param parserContext the object encapsulating the current state of the parsing process
 * @return the BeanDefinition resulting from the parsing of the supplied {@link Element}
 * @throws IllegalStateException if the bean {@link Class} returned from
 * {@link #getBeanClass(org.w3c.dom.Element)} is {@code null}
 * @see #doParse
 */
@Override
protected final AbstractBeanDefinition parseInternal(Element element, ParserContext parserContext) {
    BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition();
    String parentName = getParentName(element);
    if (parentName != null) {
        builder.getRawBeanDefinition().setParentName(parentName);
    }
    //获取自定义标签中的class,此时会调用自定义解析器,如UserBeanDefinitionParser中的getBeanClass方法
    Class<?> beanClass = getBeanClass(element);
    if (beanClass != null) {
        builder.getRawBeanDefinition().setBeanClass(beanClass);
    }
    else {
        //若子类没有重写getBeanClass方法则尝试检查子类是否重写getBeanClassName方法
        String beanClassName = getBeanClassName(element);
        if (beanClassName != null) {
            builder.getRawBeanDefinition().setBeanClassName(beanClassName);
        }
    }
    builder.getRawBeanDefinition().setSource(parserContext.extractSource(element));
    if (parserContext.isNested()) {
        // Inner bean definition must receive same scope as containing bean.
        builder.setScope(parserContext.getContainingBeanDefinition().getScope());
    }
    if (parserContext.isDefaultLazyInit()) {
        // Default-lazy-init applies to custom bean definitions as well.
        builder.setLazyInit(true);
    }
    doParse(element, parserContext, builder);
    return builder.getBeanDefinition();
}